IDO1 and cancer: It acts on a variety of tryptophan substrates, including D-tryptophan, L-tryptophan, 5-hydroxytryptophan, tryptophan, serotonin, and so on.379 IDO1 plays a role in various pathophysiological processes, such as antibacterial, antitumor defense, neuropathology, immunomodulation, and antioxidant activity.380 Recently, a large number of reports have confirmed that IDO1 was overexpressed in a variety of cancers and plays an important role in cancer immune escape.